The present invention relates to the speed ratio control of a continuously variable transmission.
Tokkai Hei 8-296708 published by the Japanese Patent Office in 1996 discloses a speed ratio control system of a continuously variable transmission (CVT) wherein the groove width of a belt pulley is varied by driving a speed ratio control valve using a step motor, and the speed ratio is made to follow a target speed ratio (transient target ratio) with a dynamic characteristic desired by the designer.
This system comprises a dynamic characteristic compensator which computes a speed ratio command value so that a speed ratio response intended by the designer is achieved, and a disturbance compensator which eliminates variation of the dynamic characteristic of the CVT and disturbances. This arrangement realizes the speed ratio response intended by the designer, and increases the stability of the CVT to dynamic characteristic variations or disturbances.
However, as the dynamic characteristic compensator is a feed forward compensator, impairment of speed change performance due to the drive speed limit of the step motor (hardware or software limiter) or the angular position limit (hardware stopper) cannot be prevented.
Although the disturbance compensator is provided, this is intended to compensate for disturbances such as parameter variations due to oil deterioration or manufacturing discrepancies (variation of time constant, i.e., response variation). The disturbance compensator cannot provide compensation when the operation of the speed change mechanism is limited as mentioned above.
Therefore, under conditions when the speed ratio target value suddenly changes discontinuously, such as when the accelerator pedal is largely depressed and a downshift is performed, or when the accelerator pedal is released and an upshift is performed, the desired speed ratio response cannot be attained.
In order to improve this situation, the dynamic characteristic compensator might be made a feedback compensator. However, if the dynamic characteristic compensator is made a feedback compensator, the speed ratio response and the response to disturbances cannot be designed independently.
That is, although the dynamic characteristic compensator is designed so that the desired speed ratio response is obtained, the speed ratio response is determined by feeding back the real speed ratio including the result compensated by the disturbance compensator, so the dynamic characteristic compensator and disturbance compensator cannot be designed independently.
It is therefore an object of this invention to decrease the deviation between the transient target ratio and real speed ratio and achieve a desired speed ratio response even in the case where operation of a speed change mechanism is limited. It is a further object of this invention to independently design the speed ratio response and response to disturbances.
To achieve the above object, this invention provides a speed ratio control system comprising: a first compensator which computes a speed ratio command value based on a final target ratio determined according to a running state, a time constant representing a predetermined dynamic characteristic and a time constant representing an estimated dynamic characteristic of a transmission, a calculating element which calculates a transient target ratio based on the final target ratio and the time constant representing the predetermined dynamic characteristic, a calculating element which calculates a correction amount of the speed ratio command value based on the difference between the transient target ratio and a real speed ratio, and an adder/subtractor which corrects the speed ratio command value based on the correction amount. The control system controls the transmission based on the speed ratio command value after correction.
According to this invention, the speed ratio control system controls the continuously variable transmission so that the real speed ratio approaches the final target ratio determined according to a running condition under the predetermined dynamic characteristic, and the real speed ratio follows the transient target ratio computed based on the predetermined dynamic characteristic and final target ratio. When the real speed ratio does not approach the final target ratio under the predetermined dynamic characteristic due to a drive speed limit or an angular position limit, etc. of a step motor, and a difference arises between the transient target ratio and real speed ratio, a correction according to the difference is applied to the speed ratio command value. In this way, the difference between the transient target ratio and real speed ratio can be reduced, so even if step motor has a drive speed limit and angular position limit, the impairment of speed ratio response performance which would otherwise occur can be suppressed.
Moreover, according to this invention, a speed ratio control system is provided having a second compensator which further compensates the speed ratio command value by a filter using approximate zeroing so that disturbances acting on the transmission are compensated. By correcting the speed ratio command value so that disturbances, such as parameter fluctuations and step out of the step motor, are compensated, the dynamic characteristic compensator which computes the speed ratio command value is not subject to these influences and the speed ratio response and responsiveness to disturbances can be designed independently.
Further, according to this invention, the speed ratio command value correction amount calculating element computes a correction amount of the speed ratio command value by passing the difference between the transient target ratio and real speed ratio via a filter whereof the stability relative to variation of a time constant representing a dynamic characteristic of the transmission, is compensated. Hence, the speed ratio response and responsiveness to disturbances can be designed independently, and even if the time constant which represents the dynamic characteristic of the CVT is incorrectly estimated, the effect of this on speed ratio control performance response can be suppressed low.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.